1. Field of the Invention
The present invention relates to an amorphous silicon device and a method of dry etching an amorphous silicon device. More particularly, the present invention relates to a method of dry etching amorphous silicon in a dry etching process by which a fine-shaping process is carried out during the production of a thin film electronic device such as a diode or transistor, in which amorphous silicon is used as the thin film material, wherein deterioration of the electrical characteristics of the amorphous silicon channel portion due to exposure to an etching plasma or etching beam during the dry etching process is prevented, and thus the electrical characteristics are not substantially damaged even when a fine-shaping process indispensable to the formation of the device is applied, and an amorphous silicon device obtained therefrom.
2. Description of the Related Art
The process for forming an element having an electrical function, such as a diode or transistor, from a semiconductor electronic material such as a single crystalline silicon is called a device formation process.
In such a device formation process, the process of shaping of a single crystalline silicon semiconductor portion or the wiring metal portion of, for example, Al and Cr, is generally called "fine shaping".
In such fine shaping, the means for removing the material beneath the site not covered by the photoresist subjected to patterning may be broadly classified into (1) the wet etching method and (2) the dry etching method.
In the wet etching method the photoresist etching portion, i.e., the subbing material not covered by the photoresist, is removed by etching with a liquid etchant such as acid or alkali, and this method is widely utilized in the prior art. Nevertheless, in general, etching with such an etching treatment agent is isotropic (i.e., etching proceeds at the same rate in all directions), and thus this method is unsuitable when the pattern must be very fine or when the cut edge of the patterning portion is to be made vertical.
The dry etching method is a new process proposed in an attempt to solve the problems of the wet etching method as described above. In a typical example of this method, an etching gas such as CF.sub.4 or CCl.sub.4 is placed in a vessel under a low pressure, and using parallel plate type electrodes, a plasma is generated by a high frequency discharge, at a certain voltage, between these electrodes. The plasma contains fluorine-active species or chlorine active species, actively reactive with the subbing material, in large amounts, and therefore, the subbing material is etched at the photoresist etching portion. Here, by appropriately selecting the bias between the electrodes, the kind of gas, and the pressure of the gas, etc., anisotropic etching is possible, and this anisotropic etching is called RIE (i.e., reactive ion etching). As anisotropic dry etching methods other than this method, there are known the ion beam etching method in which ions or atoms of an inert gas are impinged against a substrate to physically or mechanically sputter the atoms of the substrate, and the reactive ion beam etching method in which ions or atoms of a reactive gas are irradiated on a substrate.
Using a dry etching method such as RIE, the subbing material can be finely and vertically etched only at the photoresist etching portion, and at the same time, the problem of undercutting arising in the wet etching method is solved. Therefore, the shaping of a very fine or complicated pattern becomes possible.
FIGS. 3(a)-3(c) show the patterning of a typical single crystal silicon by the wet etching method and by the dry etching method. The patterning by the dry etching method shown in FIG. 3(b) shows the single crystalline silicon 9 immediately below the photo-resist 1 shown in FIG. 3(a), which is subjected to vertical patterning, and the patterning by the wet etching shown in FIG. 3(c) shows the isotropic etching of the single crystalline silicon 9, and at the same time, the undercut 11 appears at the portion immediately below the photoresist 1 of the single crystalline silicon 9.
Also, since the dry etching method does not use a liquid etchant such as acid or alkali, the process is clean and compact and easily maintained. In addition, it becomes possible to etch a material which can be etched only with difficulty by the wet etching method (e.g., Si.sub.3 N.sub.4) by using the dry etching method. Therefore, the dry etching method is now widely utilized in processing semiconductor materials, including typical single crystalline silicon.
In the dry etching method, however, since active species which are hyperactive are employed, the dangling bonds (i.e., unbonded arms of silicon) at the etching surface are increased, and consequently, a problem arises in that the electrical characteristics obtained by the dry etching method become poor. This is because an increase of the dangling bonds at the etching surface results in an etching surface which acts as an electroconductive channel, as shown in FIG. 4(a). Here, the problem is described using as an example an amorphous silicon diode having metal 10--p-type amorphous silicon 5--i-type amorphous silicon 4--n-type amorphous silicon 3--metal 10 layers formed, in this order, on the substrate 7, and the Figure shows that a defective (or electroconductive) channel portion 8 is formed on the surface of the diode etched by the dry etching method.
As a result, in an electronic device such as a diode and transistor, the off-current is increased, and thus it becomes a "leakable" device and presents a great problem in the preparation of a semiconductor device.
To solve the problems of the dry etching method, two typical methods have been proposed, in one of which the defective device is annealed in a nitrogen atmosphere at 500.degree. C. or higher, or in a vacuum. Generally, this annealing is performed for several hours in a furnace, and reduces the dangling bonds at the defective channel portion as mentioned above, to reduce the off-current of the "leakable" device and thereby improve the electrical characteristics, as is widely known and used in the art for the formation of all semiconductor materials.
In another method, the channel portion with increased dangling bonds is removed by the wet etching method or the dry etching method. The channel portion, which was made defective during the etching process, exists under the exposed portion, and therefore, after the conventional etching process (namely, the etching process by which the defect is caused), anisotropic etching is effected to remove the channel portion. FIG. 4(b) shows the shape of the amorphous silicon diode from which the defective channel 8 has been removed.
These methods however, present difficulties when applied, because the problems as described below arise when the material to be shaped is an amorphous silicon.
First, in the annealing method, as well known in the art, the amorphous silicon contains a large amount of hydrogen in the form of Si--H, due to a low temperature non-equilibrium film formation, and this bond remarkably improves the electrical characteristics of the amorphous silicon (for example, reduction of localized level density). Nevertheless, the Si--H bond is easily destroyed by heat to release hydrogen when annealed at 300.degree. C. or higher, whereby a problem arises in that the electrical characteristics become very poor.
The other method also has problems in the removing of the channel portion of the device. First, since the defective channel portion is formed during the dry etching process, even if an etching method which causes little damage is selected, it is very difficult in practice to achieve an ideal isotropic etching by cleanly removing only the defective channel portion without leaving defects on the freshly etched surface of the device. Also, since the isotropic etching process is very delicate, it is difficult to control and has a poor reproducibility. Therefore, the yield is low, and even within the same substrate, if the sizes of the etched portions are irregular, a problem arises in that variations of the characteristics of the elements occur.
For the reasons mentioned above, in the device using an amorphous silicon, an effective and improved method of forming the amorphous silicon channel portion by dry etching has not been found, and thus the degree of freedom of the design of the device structure and in the production process is very low.